D O C . 2 3 1 W A R B U R G A S R E S E A R C H E R 1 8 9 with the wall, on average, they do not have any rate of flow. On average, therefore, the molecules directly next to the stationary wall have a flow rate other than zero (apparent slippage). By an entirely similar consideration one finds that a jump in temperature must take place between the wall and the gas if a temperature gradient (heat flux) exists orthogonally to the wall. The temperature of the gas at the wall must be as prevails at a distance of 0.7 from the wall without a temperature jump. The existence of both effects was experimentally proved beyond doubt by Kundt and Warburg: important arguments that the kinetic theory of gases corresponds to reality. It was the first time that a new phenomenon was predicted on the basis of the molecular theory of heat, in particular, a phenomenon that on the basis of the conception of continuous matter was as good as excluded. If energeticists at the end of the 19th century had appreciated these arguments sufficiently, they would have hardly been able to doubt seriously the deeper legitimacy of the molecular theory. A year later the two authors found another important experimental proof arguing in favor of the kinetic theory of gas. They demonstrated that the heat capacity of mercury vapor was per mole (R = constant of the gas equation). For if mona- tomic gas molecules have no rotational energy, therefore act like material points, then the entire thermal energy of a gas will just consist of the progressive motion of its molecule, which for its part uniquely determines the pressure at a given vol- ume. This corresponds to the equation: . The proof was given by measuring the velocity of sound according to Kundt’s method. The experimental work of the next few years 1872–79 is devoted to the study of external friction, particularly the study of elastic properties of solids deformed beyond their elasticity limit. These researches may have led Warburg by analogy to one of the finest fruits of his labors, namely to the proof that the cyclic magnetiza- tion of ferromagnetic substances is connected to a loss in mechanical or electro- magnetic energy that manifests itself as hysteresis heat (1881). At that time he also found the quantitative relationship between this energy loss with the surface area of the hysteresis curve. Warburg calculated the potential energy of a permanent magnet with reference to a magnetized piece of iron as , λ [p. 824] 3 2 --R - thermal energy 3 2 --pV - 3 2 --RT - = = Φ + dV Jx------ ∂ϕ ∂x Jy------ ∂ϕ ∂y Jz------ ∂ϕ ∂z + + Jh)dV ( – = =